1. Field of the Invention
The present invention relates to a method of and apparatus for forming a colored band of graded intensity having a curved cut-off line along one edge thereof and a desired transverse pattern of graded intensity along a longitudinally extending area of a workpiece in the form of a flexible sheet of plastic interlayer material suitable for use as an interlayer in laminated safety glass. The workpiece is preferably coated by electrostatic spraying to insure that very fine droplets of dye composition form a band of color having a desired intensity pattern along its transverse dimension that may comprise a first coating portion of substantially uniform maximum intensity merging into a second coating portion of changing intensity that merges into a third coating portion of finely reduced intensity that merges into a barely perceptible cut-off line between coated and uncoated portions.
This invention is especially concerned with the partial coating of flexible interlayer material that is a component of laminated glass used in automobiles. The partial coating according to the present invention is performed in such a manner that it is unnecessary to differentially stretch partially coated interlayer sheets removed from a continuous ribbon to provide a curved cut-off line along the edge of the colored portion so that when the partly dyed interlayer is laminated to one or more curved sheets of rigid transparent material, such as glass or a recognized rigid transparent plastic substitute for glass such as polycarbonates, acrylic plastics, polyesters and rigid polyurethanes (hereinafter also referred to as glass), the cut-off line between the coated and uncoated portions will appear to have a horizontal line in the curved laminated windshield installed in an inclined relation in an automobile frame.
In electrostatic spraying of sheets of interlayer material as practiced by the prior art, a non-conductive workpiece is exposed to highly charged particles of a dye composition. A grounded shield of electroconductive material is interposed between an electrostatic spray gun and the workpiece and a manifold is located behind the outboard edge of the grounded shield and between the lower surface of the shield and the upper surface of the workpiece to provide a positive pressure of a non-reactive fluid, such as air, in the direction of the edge of the grounded shield parallel to the surface of the workpiece being coated so as to reduce the intensity of underspray that is deposited on the upper surface of the workpiece in facing relation to the shield. Grounding the shield selectively attracts many highly charged particles of dye contained in the dye composition that would otherwise mutually repel one another and deposit on the upper surface of the workpiece in the region over a wide area facing the shield and extending laterally from its inboard edge.
In the past, individual flexible sheets of polyurethane or plasticized polyvinyl acetal (preferably the butyral) were separated from a continuous ribbon of said material, and the individual sheets were supported in a given orientation and electrostatically sprayed while supported in the given orientation. It is easy to load and unload a flexible sheet from a substantially horizontal support. However, droplets of spray composition tend to form larger drops, which deposit on the surface to be coated, thus causing optical defects when a spray gun is mounted above the sheet to be coated. Applying an electrostatic spray from one side of a sheet supported obliquely reduces the chance of such defects. However, the difficulty of mounting and supporting a flexible sheet in an oblique orientation has discouraged this solution in the past.
The concept of applying paints and other tinting compositions by an electrostatic spray system has been developed. U.S. Pat. No. 3,645,477 to Cowen discloses an air atomized electrostatic spray device in which air is supplied to the device for the purpose of atomizing a liquid to be sprayed. The air is also employed for operating a self-contained electrogasdynamic power generator for charging the atomized coating material and for maintaining an electrostatic depositing field having one terminus adjacent the locus of atomization. A highly charged atomized mist is imparted to a substrate to be coated. Electrostatic spraying develops a coating of a given intensity characterized by a low transmission coefficient more rapidly than older spraying methods.
The mist so produced comprises highly charged particles that mutually repel one another during the electrostatic spray process. This mutual repulsion causes the particles to spread over a wide area en route to a substrate to be coated. Hence, the electrostatic spray process is suitable for coating an entire substrate uniformly. When an electrostatic spray is applied to an exposed portion only of a substrate, the mutual repulsion of the spray particles causes the electrostatic spray particles to deposit on a large area outside of the exposed portion to which the electrostatic spray is applied and form a pattern of gradually reducing intensity within said large area outside of said exposed portion. Even the interposition of a mask or shield, which mechanically controls the boundary in operations in which hydrolyzable salt compositions and sprays other than electrostatic sprays are applied to form coatings in the form of a band, fails to avoid extensive areas of fade-out in coatings applied by electrostatic spraying, unless the shield is grounded.
Since the details of the electrostatic spray device does not form part of the present invention and such devices are readily available commercially, the details of the spray gun or power generator for use with the spray gun will not be described in detail in this specification. However, details of a suitable electrostatic spray device may be found in U.S. Pat. No. 3,645,447 to Cowen and details of a suitable electrogasdynamic generator for such a suitable electrostatic spray device are recited in U.S. Pat. No. 3,651,354 to Cowen. The disclosures of these patents relative to an electrostatic spray device and an electrogasdynamic generator for such a device are incorporated herein by reference in order to avoid an excessively long specification.
Prior to the present invention, a need existed for coating equipment that combined an easy means for loading and unloading a flexible sheet of interlayer material and for supporting said flexible sheet in an oblique orientation in alignment with a shield support system to facilitate the application of a spray composition electrostatically onto an elongated area of the flexible sheet.
2. Description of the Prior Art
A search of the prior art failed to find an anticipation of loading and unloading a flexible sheet of interlayer material in one orientation most suitable for loading and unloading and for supporting the flexible sheet on a vacuum platen in alignment with a platen mask carried by a shield support system in an oblique orientation for coating an exposed elongated area of the flexible sheet through an opening in said platen mask by electrostatic spray means disposed laterally from the elongated area. The patents reported in the search described vacuum means to handle rigid, fragile materials such as glass sheets. U.S. Pat. No. 3,399,885 to Agitt et al mentions the use of suction attachment devices to secure a hold on a glass sheet stacked on a suitable support to lift each sheet from the stack and place it on a transfer device which delivers it to a subsequent sheet cutting or processing machine.
U.S. Pat. No. 3,411,639 to Dryon uses suction cups mounted on a mobile frame to help clamp means to support glass sheets on frames that pivot about pivot means disposed to one side of a conveyor for transferring glass sheets from the conveyor and swung to a position past the vertical where the glass sheets can be stacked or packed.
U.S. Pat. No. 3,679,073 to Malburet discloses the use of vacuum nozzles to support a glass sheet while the latter is tilted.
U.S. Pat. No. 3,682,329 to Dean uses a vacuum braking device to stop a glass sheet before it contacts retractable stops on the bottom of a platen along which the glass sheet slides.
U.S. Pat. No. 3,765,550 to Tausheck discloses a dolly for lifting and tilting large glass sheets which are partially supported by vacuum cups during their transfer from a case to a tilting cutting table or to a truck or to a window opening at a building site.
U.S. Pat. No. 3,790,003 to Tausheck discloses a tiltable table for cutting glass sheets where a vacuum cup is applied to an edge of the glass sheet and the vacuum cup moved by a winch to transfer a glass sheet from a cutting position to a glass sheet removal position over rubber rollers.
U.S. Pat. No. 3,966,048 to Nunes et al uses suction to transfer a glass sheet from a conveyor to an inspection station.
A copending patent application of Dennis S. Postupack, U.S. Ser. No. 868,013, filed Jan. 9, 1978 for Method of Forming Graded Shade Band on Substrate, now U.S. Pat. No. 4,138,284, discloses a method and apparatus for applying a shade band by electrostatically spraying a dye composition along an axis extending in a direction oblique to a plane of support for a flexible sheet of interlayer material. The plane of support for the flexible sheet may be substantially horizontal or oblique. A horizontal plane of support is convenient for loading and unloading, but not the best orientation suitable for applying a spray composition. An oblique plane of support is convenient for applying a spray composition, but is inconvenient for loading and unloading.